What is the Iron-Carbon Phase Diagram?

Iron-Carbon Phase Diagram

The iron-carbon phase diagram is widely used to understand the different phases of steel and cast iron. Both steel and cast iron are a mix of iron and carbon. Also, both alloys contain a small number of trace elements.

The graph is quite complex but since we are limiting our exploration to Fe3C, we will only be focusing up to 6.67 weight percent of carbon.

This iron-carbon phase diagram is plotted with the carbon concentrations by weight on the X-axis and the temperature scale on the Y-axis.

Fig. shows, the Fe-C equilibrium diagram in which various structures (obtained during heating and cooling), phases, and microscopic constituents of various kinds of steel and cast iron are depicted. The main structures, the significance of various lines, and critical points are discussed as under.

Iron-Carbon Phase Diagram

Structures in Fe-C-diagram

The main microscopic constituents of iron and steel are as follows:

  • Austenite
  • Ferrite
  • Cementite
  • Pearlite

1. Austenite

Austenite is a solid solution of free carbon (ferrite) and iron in gamma iron. On heating the steel, after upper critical temperature, the formation of structure completes into austenite which is hard, ductile, and non-magnetic.

It is able to dissolve a large amount of carbon. It is in between the critical or transfer ranges during the heating and cooling of steel. It is formed when steel contains carbon up to 1.8% at 1130°C. On cooling below 723°C, it starts transforming into pearlite and ferrite. Austenitic steels cannot be hardened by usual heat treatment methods and are non-magnetic.

2. Ferrite

Ferrite contains very little or no carbon in iron.  It is the name given to pure iron crystals which are soft and ductile. The slow cooling of low carbon steel below the critical temperature produces a ferrite structure. Ferrite does not harden when cooled rapidly. It is very soft and highly magnetic.

3. Cementite

Cementite is a chemical compound of carbon with iron and is known as iron carbide (Fe3C).  Cast iron having 6.67% carbon is possessing the complete structure of cementite. Free cementite is found in all steel containing more than 0.83% carbon. It increases with an increase in carbon % as reflected in the Fe-C  Equilibrium diagram. It is extremely hard.

The hardness and brittleness of cast iron are believed to be due to the presence of cementite.  It decreases tensile strength.  This is formed when the carbon forms definite combinations with iron in form of iron carbides which are extremely hard in nature. The brittleness and hardness of cast iron are mainly controlled by the presence of cementite in it. It is magnetic below 200°C.

4. Pearlite

Pearlite is a eutectoid alloy of ferrite and cementite. It occurs particularly in medium and low carbon steels in the form of a mechanical mixture of ferrite and cementite in the ratio of 87:13.  Its hardness increases with the proportion of pearlite in ferrous material.

Pearlite is relatively strong, hard, and ductile, whilst ferrite is weak, soft, and ductile. It is built up of alternate light and dark plates.

These layers are alternately ferrite and cementite. When seen with the help of a  microscope, the surface has an appearance like a pearl, hence it is called pearlite. Hard steels are mixtures of pearlite and cementite while soft steels are mixtures of ferrite and pearlite.

As the carbon content increases beyond 0.2%  in the temperature at which the ferrite is first rejected from austenite drop until, at or above  0.8%  carbon, no free ferrite is rejected from the austenite. This steel is called eutectoid steel, and it is a pearlite structure in composition.

As iron having various % of carbon (up to 6%) is heated and cooled,  the following phases representing the lines will tell about the structure of iron,  how it charges.

Significance of Transformations Lines

1. Line ABCD

The line  ABCD  tells that above this line melting has been completed during heating the iron.  The molten metal is purely in the liquidus form.  Below this line and above line  AHJECF the metal is partially solid and partially liquid.

The solid metal is known as austenite. Thus the line  ABCD  represents temperatures at which melting is considered as completed. Beyond this line, metal is totally in a molten state. It is not a horizontal line the melting temperature will vary with carbon content.

2. Line AHJECF

This line tells us that metal starts melting at this temperature. This line is not horizontal and hence the melting temperatures will change with carbon content. Below this line and above line  GSEC, the metal is in solid form and having an austenite structure.

3. Line PSK

This line occurs near 723°C  and is a  horizontal line and is known as a lower critical temperature line because the transformation of steel starts at this line.  Carbon % has not to affect on it that means steel having different % of carbon will transform at the same temperature.

The range above the line up to GSE  is known as the transformation range. This line tells us the steel having carbon up to 0.8%  up to 0.8%  will start transforming from ferrite and pearlite to austenite during heating.

4. Line  ECF

It is a line at temperature 1130°C  which tells that for cast iron having % of C from  2% to  4.3%.  Below this line and above line  SK,  Cast iron will have austenite + ledeburite and cementite +  ledeburite.

FAQs.

What is the iron carbon diagram?

The Fe – C diagram (also called the iron-carbon phase or equilibrium diagram) is a graphic representation of the respective microstructure states of the alloy iron-carbon (Fe-C) depending on temperature and carbon content. To explain this diagram, an introduction about metal structures and pure iron must be done.

How do you read an iron carbon diagram?

Why iron carbon diagram is called equilibrium diagram?

The iron carbide is called the metastable phase. Therefore, the iron-iron carbide diagram even though technically represents metastable conditions, can be considered as representing equilibrium changes, under conditions of relatively slow heating and cooling.

What are the phases in iron carbon phase diagram?

For the iron-carbon phase diagram, the phase fields of interest are ferrite, cementite, austenite, ferrite + cementite, ferrite + austenite, and austenite + cementite phase fields.

What is the carbon equilibrium diagram?

The Iron carbon equilibrium diagram (also called the iron-carbon phase diagram) is a graphic representation of the respective microstructure states of the alloy iron-carbon (Fe-C) depending on temperature and carbon content.

What is the percentage of carbon in eutectoid steel?

Steel containing 0.8% C is known as eutectoid steel. The equilibrium microstructure of eutectoid steel obtained at room temperature is pearlite.

What is mild steel?

Mild steel is a ferrous metal made from iron and carbon. It is a low-priced material with properties that are suitable for most general engineering applications. Low carbon mild steel has good magnetic properties due to its high iron content; it is therefore defined as being ‘ferromagnetic’.

What is steel made of?

steel, alloy of iron and carbon in which the carbon content ranges up to 2 percent (with a higher carbon content, the material is defined as cast iron). By far the most widely used material for building the world’s infrastructure and industries, it is used to fabricate everything from sewing needles to oil tankers.

What is the significance of a0 temperature in iron iron carbide diagram?

The A1 line is the eutectoid-temperature line and is the lowest temperature at which f.c.c. iron can exist under equilibrium conditions. Just above A1 line, the microstructure consists of approximately 25 percent austenite and 75 percent ferrite.

What is critical temperature in iron carbon diagram?

Thus, it is the temperature corresponding to gamma + alpha/gamma phase boundary for hypo-eutectoid steel and is a function of carbon content of the steel, as it decreases from 910 deg C at 0 % C to 727 deg C at 0.76 % C. It is also called the upper critical temperature of hypo-eutectoid steels.

What is eutectoid temperature in iron carbon diagram?

The eutectoid reaction describes the phase transformation of one solid into two different solids. In the Fe-C system, there is a eutectoid point at approximately 0.8wt% C, 723°C. The phase just above the eutectoid temperature for plain carbon steels is known as austenite or gamma.

What is the importance of iron carbon diagram?

The iron-carbon phase diagram is widely used to understand the different phases of steel and cast iron. Both steel and cast iron are a mix of iron and carbon. Also, both alloys contain a small amount of trace elements.

Is carbon a FCC?

Carbon is more soluble in the FCC phase, which occupies area “γ” on the phase diagram, than it is in the BCC phase. The percent carbon determines the type of iron alloy that is formed upon cooling from the FCC phase, or from liquid iron: alpha iron, carbon steel (pearlite), or cast iron.

How do you know if its BCC or FCC?

The most direct difference between FCC and BCC crystals is in the atomic arrangements. The face-centered cubic structure has an atom at all 8 corner positions, and at the center of all 6 faces. The body-centered cubic structure has an atom at all 8 corner positions, and another one at the center of the cube.

Is steel a BCC or FCC?

The alpha phase is called ferrite. Ferrite is a common constituent in steels and has a Body Centred Cubic (BCC) structure [which is less densely packed than FCC].

What is the iron carbon equilibrium?

Under equilibrium conditions, pro-eutectoid ferrite will form in iron-carbon alloys containing up to 0.8 % carbon. The reaction occurs at 910°C in pure iron, but takes place between 910°C and 723°C in iron-carbon alloys.

What is perlite in iron carbon diagram?

Pearlite is the eutectoid mixture of cementite and ferrite. Iron carbon equilibrium diagram: The phase diagram of Fe-Fe3C is not a true equilibrium because the iron carbide is an unstable phase that after prolonged heat treatment decomposes into iron and carbon (the graphite form).

What is ferrite in iron carbon diagram?

Ferrite is known as α solid solution. It is an interstitial solid solution of a small amount of carbon dissolved in α (BCC) iron. The maximum solubility is 0.025 % C at 723C and it dissolves only 0.008 % C at room temperature. It is the softest structure that appears on the diagram.

Which is almost pure iron phase?

Pure iron (α-iron, or ‘ferrite’) undergoes a change in crystal structure when heated above 910 °C, forming γ-iron, or ‘austenite’.

Why ACM line is steeper than A3 line?

Acm line is much steeper than A3 line, which though, means that the amount of proeutectoid cementite in commercial steels is very small, but it also means that heating, too much high temperatures, has to be done to dissolve this cementite for complete homogenisation of austenite.